Concerns over the environmental consequences of burning fossil fuels have led to an increasing use of renewable energy generated from sources such as solar and wind. The intermittent and varied nature of such renewable energy sources, however, has made it difficult to fully integrate these energy sources into electrical power grids and distribution networks. A solution to this problem has been to employ large-scale electrical energy storage (EES) systems, which systems are widely considered to be an effective approach to improve the reliability, power quality, and economy of renewable energy derived from solar or wind sources.
Among the most promising large-scale EES technologies are redox flow batteries (RFBs). RFBs are special electrochemical systems that can repeatedly store and convert megawatt-hours (MWhs) of electrical energy to chemical energy and chemical energy back to electrical energy when needed.
In simplified terms, an electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. An electrochemical cell comprises two half-cells. Each half-cell comprises an electrode and an electrolyte. The two half-cells may use the same electrolyte, or they may use different electrolytes. In a full electrochemical cell, species from one half-cell lose electrons (oxidation) to their electrode while species from the other half-cell gain electrons (reduction) from their electrode. Plural electrochemical cells electrically connected together in series within a common housing are generally referred to as an electrochemical “stack.”
A redox (reduction/oxidation) flow battery (RFB) is a special type of electrochemical system in which electrolyte containing one or more dissolved electroactive species flows through electrochemical cells. A common RFB electrochemical cell configuration comprises two opposing electrodes separated by an ion exchange membrane or other separator, and two circulating electrolyte solutions, referred to as the “anolyte” and “catholyte.” The energy conversion between electrical energy and chemical potential occurs instantly at the electrodes once the liquid electrolyte begins to flow through the cells. The advantages of RFBs include short response time, low self-discharge, long life-time, and independent tunable power and storage capacity, which make it possible to independently scale up the electricity storage capacity and power generation capacity.
The construction and operation of RFBs may be better understood with reference to FIGS. 1A and 1B. The illustrated RFB includes an electrochemical cell 100, a catholyte tank 15, filled with liquid catholyte 20, and an anolyte tank 30, filled with liquid anolyte 35. The RFB 10 operates by circulating the catholyte 20 and anolyte 35 into the electrochemical cell 100, which then operates in order to discharge or store as directed by power and control element 50 which is in electrical communication with the electrochemical cell 100.
While a single electrochemical cell 100 is illustrated in FIG. 1A, it will be appreciated that multiple electrochemical cells, assembled into a stack, can also be used in an RFB. An example of such a stack is illustrated in FIGS. 2A and 2B.
In one mode (sometimes referred to as the “charging” mode), the power and control element 50, connected to a power source, operates to store electrical energy as chemical potential in the catholyte 20 and anolyte 35. The power source can be any power source known to generate electrical power, include renewable power sources, such as wind, solar, and hydroelectric. Traditional power sources, such as combustion, can also be used.
In a second (“discharge”) mode of operation, the RFB 10 is operated to transform the chemical potential stored in the catholyte 20 and anolyte 35 into electrical energy that is then discharged on demand by power and control element 50, which supplies an electrical load. FIG. 1A illustrates the flow of electrons (“e−”) through the RFB 10 in discharge mode. The operation of the RFB 10 in charging mode is essentially the opposite of operation in discharge mode.
The operation of RFBs, such as that illustrated in FIG. 1A, are well known to those of skill in the art.
Referring now specifically to FIG. 1B, a close up view of the electrochemical cell 100 as illustrated in FIG. 1A is depicted. The electrochemical cell 100 includes a positive electrode 105, a negative electrode 115, a catholyte channel 110, an anolyte channel 120, and an ion transfer membrane 125 separating the catholyte channel 110 and the anolyte channel 120. The ion transfer membrane 125 separates the electrochemical cell 100 into a positive side 130 and a negative side 135. Selected ions (e.g., H+) are allowed to transport across the ion transfer membrane 125 as part of the electrochemical charge and discharge process. Electrons flow outside the electrochemical cell 100 and through the power and control element 50.
Also illustrated in FIG. 1B are inlets and outlets configured to allow integration of the electrochemical cell 100 into the RFB 10 electrolyte flow systems: a catholyte inlet 107 and a catholyte outlet 109, as well as an anolyte inlet 117 and an analyte outlet 119.
Referring back to FIG. 1A, the electrochemical cell 100 is integrated into the RFB such that during operation, the catholyte 20 flows through the catholyte delivery channel 55, aided by a first pump 25, into the catholyte channel 110 of the electrochemical cell 100. Similarly, the anolyte 35 flows through an anolyte delivery channel 65, with the aid of a second pump 40, and into the anolyte channel 120 of the electrochemical cell.
After operation of the electrochemical cell 100 to either discharge or store electrical energy, the catholyte flows from the catholyte channel 110 through a catholyte return channel 60 back to the catholyte tank 15. Similarly, the anolyte 35 flows from the anolyte channel 120 in the electrochemical cell 100 through the anolyte return channel 70 to the anolyte tank 30.
The operation of the RFB 10 continues as needed to discharge or store electrical energy.
To obtain high voltage/power systems, a plurality of single electrochemical cells may be assembled together in series to form a stack of electrochemical cells (referred to herein as a “stack,” a “cell stack,” or an “electrochemical cell stack”). Several cell stacks may then be further assembled together to form a battery system. A MW-level RFB system can be created and generally has a plurality of cell stacks, with each cell stack having more than twenty electrochemical cells.
The combination of a plurality of electrochemical cells is illustrated in FIG. 2A, which shows a schematic illustration of the structure of a four-cell stack assembly that could be used in an RFB.
Referring to FIG. 2A, an electrochemical cell stack 200 as can be integrated into an RFB, for example to replace electrochemical cell 100 illustrated in FIG. 1A, is depicted schematically. The cell stack 200 includes a plurality of electrochemical cells, similar to the electrochemical cell 100 illustrated in FIGS. 1A and 1B; however, the electrochemical cells are arranged fluidically in parallel and electrically in series in order to allow current to pass across the entire stack from a positive electrode 230 to a negative electrode 235.
The cell stack 200 includes end plates 240 and 245 in order to mechanically press the structure together to provide structural integrity between the layers of the cell stack 200. For example, tie rods spanning the stack 200 can be used to connect the end plates 240 and 245 and apply a force between them sufficient to maintain the structure of the cell stack 200.
Each cell of the cell stack 200 includes a catholyte channel 110 and an anolyte channel 120, separated by an ion transfer membrane 125. In between the individual cells is a bipolar plate electrode that is able to hold a positive charge on one side and a negative charge on the opposite side of the material. Bipolar electrodes 205 are also used in between the positive electrode 230 and the cell stack, as well as in between the negative electrode 235 and the cell stack. The bipolar plate electrodes 205 can be the same or different material across the cell stack 200.
The cell stack 200 operates in an RFB by passing catholyte and anolyte through their respective channels (110 and 120). The catholyte is delivered to the cells by a catholyte delivery manifold 250, which provides liquid communication between the catholyte tank (e.g., catholyte tank 30 in FIG. 1A) and the cell stack 200. As depicted in FIG. 2A, the catholyte delivery manifold 250 is in liquid communication with each of the catholyte channels 110 within the cell stack 200. Similarly, an anolyte delivery manifold 255 delivers anolyte to the anolyte channels 120 of the cell stack 200 from the anolyte tank of the RFB (e.g., anolyte tank 30 in FIG. 1A).
On the return side of the cell stack 200 a catholyte return manifold 260 collects catholyte from the catholyte channels 110 and returns them to the catholyte tank (e.g., catholyte tank 30 in FIG. 1A). Similarly, the anolyte return manifold 265 collects anolyte from the anolyte channels 120 and returns the collected anolyte to the anolyte tank (e.g., anolyte tank 30 in FIG. 1A).
The cell stack 200 illustrated in FIG. 2A is referred to herein as a “U-shaped” stack because the anolyte flows to and returns from the cell stack 200 on the same side of the cell stack (the left-hand side of FIG. 2A). An alternative configuration, referred to herein as a “Z-shaped” stack, is illustrated in FIG. 2B, wherein the anolyte and catholyte flow to the cell stack 200 from one side and flow away from the cell stack 200 on the opposite side.
Despite their advantages, one issue associated with RFBs is inhomogeneous electrical performance of individual electrochemical cells within a cell stack. Accordingly, there is a need to improve the reliability of cell stacks to ensure long-term operation of RFBs.